In April 2020, the driver of an HGV hauling nitric acid noticed vapour coming from the tanker barrel in the area of the pressure/vacuum relief valve. He stopped at the roadside and rang the emergency services. Police closed the road and it remained closed until the tanker barrel had been fitted with a replacement valve. The tanker continued the short distance to its intended destination where it was safely discharged. There were no injuries or property damage, although the road closure will have affected people and businesses nearby.
During what should have been a momentary venting operation, the vacuum relief element of the valve became stuck in an open position, allowing an uncontrolled escape of hazardous vapour.
Following an investigation by Cleveland Police and the HSE, it was determined that the relief valve fitted to the tanker barrel had been modified in a way that made it unsafe. The flanged bolt on the end of the valve stem had been removed, and a nut had been welded to the valve cap. The end of the valve stem caught on the thread inside the nut, preventing the vacuum relief element of the valve from closing.
It is likely that this modification was made when the valve was removed for routine servicing. Some valve designs require a special tool to dismantle. The addition of the nut allows the valve cap to be unscrewed without using a special tool. Some valves fitted to other road tankers operated by the same haulage contractor were found to have the same potentially dangerous modification.
The HSE investigation also noted:
- The removal of the vacuum valve stem top bolt could allow the valve to open so far that the stem fouls on the valve cap itself. This will also prevent the valve from closing
- The locking screw in the side of the valve body had not been replaced after the valve was reassembled after servicing. This locking screw prevents the valve cap rotating, which will change the pressure-retaining capability of the valve
- The valve was being serviced less frequently than suggested by the manufacturer
- The valves vacuum protection function was not being routinely tested before refitting
The valve shown is manufactured by Fort Vale Engineering Ltd (who are not implicated in any way). Unsafe modifications may also be applied to similar devices by other manufacturers.
HSE adds: Anyone working on relief valves should be competent to do so. Valves should be serviced following the valve manufacturer’s guidance. Valves should not have temporary nor permanent modifications in order to bypass the need for special tools to carry out the work. Where a modification is felt necessary, the valve manufacturer or similar competent person should be consulted.
A link to the original notice is below.